Compound semiconductor devices, in particular, nitride semiconductor devices are actively developed as high-withstand-voltage and high-power semiconductor devices by utilizing their characteristics such as a high saturation electron velocity and a wide band gap. Many reports have been made on field-effect transistors, in particular, HEMTs (High Electron Mobility Transistors) as the nitride semiconductor devices. In particular, an AlGaN/GaN HEMT using GaN as an electron transit layer and using AlGaN as an electron supply layer has been drawing attention. In the AlGaN/GaN HEMT, a distortion resulting from a difference in lattice constant between GaN and AlGaN occurs in AlGaN. Owing to piezoelectric polarization caused by the distortion and spontaneous polarization of AlGaN, high-concentration two-dimensional electron gas (2DEG) is obtained. Therefore, a high-withstand-voltage and high-power can be realized.
In recent years, HEMTs have excellent high-speed characteristics and are thus applied to signal processing circuits of optical communication systems, other high-speed digital circuits and so on. The HEMTs have excellent low-noise characteristics and are thus expected to be applied to amplifiers in a microwave or millimeter-wave band.
On the other hand, to improve the high-frequency characteristics of the HEMTs, it is necessary to increase the value of a cutoff frequency (amplification limit frequency) fT of current gain that is the upper limit of the frequency of the amplification relating to the current gain of a transistor. For the increase, it is necessary to increase the value of a mutual conductance gm that is an element parameter relating to the amplification factor of an element and to decrease the capacitance between a gate electrode and a source electrode by reducing the gate length.
It is particularly necessary to decrease the parasitic capacitance due to an interlayer insulating film (protective film) around a gate electrode so as to prevent the high-frequency characteristics from worsening even when the HEMTs are integrated (into a MMIC, for instance). To decrease the parasitic capacitance, it is effective to reduce the dielectric constant of the interlayer insulating film and to remove the interlayer insulating film around the gate electrode.
For example, in Patent Documents 1 to 4, the interlayer insulating film ground the gate electrode is removed as follows.
First, a filled material layer is formed around the gate electrode, and then an interlayer insulating film is formed so as to cover the entire surface. Next, connection holes are formed in the interlayer insulating film so as to expose end portions of the filled material layer. Then, the filled material layer is dissolved and removed through the connection holes. Thus, the interlayer insulating film is formed so that a cavity is formed around the gate electrode.    Patent Document 1: Japanese Laid-open Patent Publication No. 2004-95637    Patent Document 2: Japanese Laid-open Patent Publication No. 2006-210499    Patent Document 3: Japanese Laid-open Patent Publication No. 5-335343    Patent Document 4: Japanese Laid-open Patent Publication No. 2009-272433
By the methods of Patent Documents 1 to 4, however, the interlayer insulating film around the gate electrode can be made into the cavity but the interlayer insulating film remains above the source electrode and the drain electrode, so that complete decrease of the parasitic capacitance cannot be achieved. This is because if the filled material is formed also on the source electrode and the drain electrode and then dissolved and removed, a part that supports the interlayer insulating film is lost and the remaining interlayer insulating film falls onto the electrodes.
As described above, in the prior art, it is difficult to make the interlayer insulating film into the cavity at a maximum to decrease as much as possible the parasitic capacitance, bringing about a problem of inhibiting the improvement in maximum operating frequency.